Reducing the effects of residual magnetism in electrical apparatus



March 11, 1930. H. L. TANNER REDUCING THE EFFECTS OF RESIDUAL MAGNETISM IN ELECTRICAL APPARATUS Filed Nov. 6, 1924 D. C. SUPPLY 3 PHA E K/EiYIOR. WMX

ORNEYS.

Patented Mar. 11, 1930 UNITED STATES PATENT OFFICE ASSIGNOR TO FORD INSTRUMENT YGRK, A GORPGBATION OF NEW YORK REDUCING THE EFFECTS OF RESIDUAL MAGHETISM IN ELECTRICAL APPARATUS Application filed November 6, 1924.

This invention relates to reducing the effects of residual magnetism in electrical apparatus, particularly of types in which residual magnetism or hysteresis affects the operation of the apparatus aside from power losses.

Apparatus of this character in which the efi'ects of residual magnetism are particularly objectionable is found in the receivers of signal transmission systems, especially of the types in which rotors or the like are caused to follow the movements of rotating fields the poles of which follow the movements of the transmitters. In such apparatus as regions of the structure in which the rotating field is produced are successively energized, the residual magnetism in previously energized regions of the structure in conjunction with the newly energized regions will cause the rotors to take up positions due to the resultant efiect of the regions which are in any degree effective. In other words, the rotating fields are produced by changes in flux density due to changes in current density, but on account of residual magnetism the flux density at any region is more or less than the value which should be produced by the given current density depending upon whether the density is decreasing or increasing. This causes errors in readings since the indicators associated with the rotors do not occupy the exact positions that they should have in accordance with the electromotive forces applied to the windings of the receivers as de termined by the transmitting instruments.

It has been proposed to reduce hysteresis losses by superimposing an alternating magnetic field upon a hysteresis cycle that is produced by a slowly varying direct current field. Some experiments have been performed along these lines in which the alternating magnetic field has been produced by passing alternating current through coils separate from those through which the main energizing current passes, so that in efiect hysteresis has been reduced by superimposing a rapidly reversing magneto-motiveforce upon a slowly varying one.

It is an object of the present invention to reduce the effects of residual magnetism in Serial No. 248,295.

electrical apparatus in which such effects are detrimental from the point of View of accuracy or the like, by superimposing a fluctuating electromotive-torce either alternating or pulsating upon the electromotive-torce which is depended upon to produce the operation of the apparatus in its intended manner.

The particular nature of the invention, as well as other objects and advantages thereof, will appear most clearly from a description of certain preferred embodiments which for purpose of illustration will be shown and described in connection with a transmission system oi the character shown in my copending aoplication, Serial No. 724,912, filed July 8, 1924.

In the accompanying drawing, Fig. 1 shows an embodiment of the invention in which residual magnetism is reduced by an alternating electromotiveiorce derived from an independent source and Fig. 2 shows an embodiment in which the alternating electromotive-force is derived from the same source as the electromotive-force which performs the functions of the transmission system.

Referring particularly to Fig. 1, T indicates generally the transmitter and R the re ceiver of the system. An armature 1 consisting of windings 2 and a commutator 3 is mounted to rotate between suitable magnetic poles which have been omitted for the sake of simplicity. Engaging the commutator at points 120 apart are three brushes, 4, 5 and 6, mounted on but insulated from a ring 7 adapted to be rotated by a pinion 8 to shift the brushes with relation to the commutator which it will be understood rotates with respect to the brushes, being driven by a suitable source of power.

From the brushes l, 5 and 6, conductors 9, 10 and 11 respectively lead to the coils 12 of the receiver R, which constitutes a threephase stator, the points of connection of the conductors being designated 13, 1% and 15 respectively. hen the armature 1 is rotating certain potential conditions are set up at the brushes 4, 5 and 6. which set up at the points 13, 14 and 15, difi'erences of potential corresponding to the instantaneous values in an alternating current circuit. These potential differences at the coils of the receiver set up a magnetic field which acts on a rotor 16 energized by direct current passing through the winding 17 of the rotor, being supplied thereto through brushes and slip rings 18 and 19 respectively. Rotation of the brush ring 7 of the transmitter will change the potential conditions at the brushes and consequently at the points 13, 14 and 15, thereby producing a rotating magnetic field in the receiver which causes the rotor 16 to assume different positions.

Owing however to the residual magnetism in the field structure of the stator of the receiver, the rotor 16 will not actually occupy the position which is should theoretically occupy in accordance with the potential conditions in the receiver coils, but will lag behind or be displaced from its true position, thus causing errors in the indications of the receiver which it will be understood is intended to be provided with suitable devices by which the position of its rotor may be determined in accordance with the corresponding position of the brush ring of the transmitter.

In accordance with the present invention provision is made for superimposing upon the electromotive-force in conductors 9, 10 and 11 a fluctuating electromotive-force of suitable frequency. As shown herein this electromotive-force is an alternating one, being impressed on conductors 9, 10 and 11 by secondaries 20, 21 and 22 respectively of transformers, the primaries 23, 24 and 25 of which are supplied from a suitable threephase source by conductors 26, 27 and 28, the other ends of the primaries being connected by a common conductor 29. Due to the effect of the alternating electromotive-force which is impressed on each of the main conductors 9, 10 and 11, the residual magnetism in any region of the field structure of the receiver following a period of energization will be greatly reduced by the following cycle or cycles of the superimposed alternating current in the windings of the receiver. The rotor of the receiver will therefore assume substantially the position which it should theoretically occupy in accordance with the potential conditions in the coils of the receiver instead of being displaced by the effect of the residual magnetism. The accuracy with which readings may be obtained will therefore be increased without any disadvantageous efiects due to the presence of the alternating electromotive-force in the circuits of the system.

As a modification of the arrangement of Fig. 1 the alternating electromotive-force may be obtained from the armature of the transmitter, as shown in Fig. 2, in which the elements corresponding to those of Fig. 1 are similarly numbered. In this case the armature is provided with slip rings 30, 31 and 32 connected by conductors 33, 34 and 35 respectively with the windings of the armature at points 36, 37 and 38, respectively, spaced 120 apart. From the slip rings 30, 31 and 32 current passes through brushes 39, 40 and 41 respectively to the primaries 23, 24 and 25 of the transformers. the secondaries 20, 21 and 22 of which are connected in the conductors 9, 10 and 11 respectively as in Fig. 1. The operation of this embodiment of the invention is the same as previously described as far as the reduction of residual magnetism in the receiver is concerned and consequent accuracy of readings. It has the advantage that both the direct electromotive-force for operating the transmission system and the alternating electromotive-force for reducing the efiects of residual magnetism are generated in the same machine, which may be of advantage in case a separate three-phase supply is not available.

\Vhile the invention has been shown and described in connection with a particular form of transmission system, it will be understood that it may be applied to other systems, and in fact, to other kinds of electrical apparatus in which the effects of residual magnetism should be overcome in so far as they may afiect the operation of the apparatus aside from the question of power losses.

I claim 1. In a transmission system, a transmitter comprising a power driven armature, a commutator connected thereto, a set of movable brushes coacting with the commutator, means for adjusting the position of said brushes to vary the potential conditions thereat, a receiver motor, conductors between said brushes and the field of the receiver motor, means associated with said conductors for introducing a fluctuating electromotive force thereto to reduce the efiect of residual magnetism in the field of the motor, and an armature for said motor relatively movable to- 1 said field in accordance with the electromotive force produced by said transmitter.

2. In a transmission system, a transmitter comprising a power driven armature, movable brushes coacting with said armature to produce polyphase potentials of variable frequency, means for moving said brushes, a polyphase receiver, having an armature operable in accordance with the potentials, conductors to transmit said potentials to said receiver, and a transformer associated with said conductors to introduce an electromotive force of relatively high constant frequency, to said conductors to reduce the hysteresis effects in the receiver.

3. In a transmission system, a transmitter comprising a power driven armature, a commutator connected thereto, a set of slip rings connected thereto, a set of brushes coactin with the commutator, means to move sai brushes to produce polyphase potentials of variable frequency, a polyphase receiver motor comprising a field and armature, conductors between said brushes and said receiver whereby the armature is moved in accordance with the potentials set up at said brushes, brushes coacting with said sli rings, and means connected with the latter rushes for introducing an alternating electromotive force of relatively high constant frequency to said conductors to reduce the residual magnetism in the receiver.

In testimony whereof I affix my signature.

HARRY L. TANNER. 

